title: Apo states of calmodulin and CaBP1 control CaV1 voltage-gated calcium channel function through direct competition for the IQ domain.
abstract: In neurons, binding of calmodulin (CaM) or calcium-binding protein 1 (CaBP1) to the CaV1 (L-type) voltage-gated calcium channel IQ domain endows the channel with diametrically opposed properties. CaM causes calcium-dependent inactivation and limits calcium entry, whereas CaBP1 blocks calcium-dependent inactivation (CDI) and allows sustained calcium influx. Here, we combine isothermal titration calorimetry with cell-based functional measurements and mathematical modeling to show that these calcium sensors behave in a competitive manner that is explained quantitatively by their apo-state binding affinities for the IQ domain. This competition can be completely blocked by covalent tethering of CaM to the channel. Further, we show that Ca(2+)/CaM has a sub-picomolar affinity for the IQ domain that is achieved without drastic alteration of calcium-binding properties. The observation that the apo forms of CaM and CaBP1 compete with each other demonstrates a simple mechanism for direct modulation of CaV1 function and suggests a means by which excitable cells may dynamically tune CaV activity.
authors.affiliation: Cardiovascular Research Institute, University of California, San Francisco, CA 94158-9001, USA.
authors.forename: Felix
authors.initials: F
authors.lastname: Findeisen
authors.forename: Christine H
authors.initials: CH
authors.lastname: Rumpf
authors.forename: Daniel L
authors.initials: DL
authors.lastname: Minor
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000378:metabolism*; D020746:Calcium Channels, L-Type / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D002147:Calmodulin / Q000378:metabolism*; D006801:Humans; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D051381:Rats; D014981:Xenopus
chemical_list: D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D002135:Calcium-Binding Proteins; D002147:Calmodulin; C085577:Ca2+-binding protein-1; D002118:Calcium
keywords: C-lobe; C-terminal lobe; CDI; CaBP1; CaM; ITC; N-lobe; N-terminal lobe; calcium sensor proteins; calcium-binding protein 1; calcium-dependent inactivation; calmodulin; electrophysiology; isothermal titration calorimetry; mathematical models; voltage-gated calcium channel